The present invention relates to improvements in combined turbo-molecular pumps.
As a prior art type of a turbo-molecular pump, there is described in Japanese patent publication No. 47-33446, a turbo-molecular pump whose outlet port is coaxially disposed with a thread-type pump.
The turbo-molecular pump of this type has the advantages that relatively low of the pumping speed, which is a drawback of the thread-type pump, is improved and its processing is simplified by utilizing the higher pumping speed that is a characteristic of the turbo-molecular pump and by providing fully compressed gas which is supplied from the inlet port to the thread-type pump.
However, this type of turbo-molecular pump can be operated at a high compression ratio and at a throughput which corresponds to the throughput of the thread-type pump when the turbo-molecular pump is operated in a high vacuum range, but in the range of 10.sup.-2 to 1 Torr, the efficiency of the turbo-molecular pump is normally extremely reduced so that the compression ratio thereof can not be improved. Thus, as shown in FIG. 1, the diminished throughput, which is a disadvantage of the thread-type pump, becomes apparent when operating in the range 10.sup.-2 to 1 Torr so that the pumping speed is rapidly reduced. Moreover, in this vacuum range, the efficiency of the rotary type pump is also at a low level. In another prior art type of turbo-molecular pump as described in Japanese patent publication No. 47-33447, alternately arranged rotary circular plates and stationary circular plates having an upper and a lower reverse spiral thereon are employed in place of the thread-type pump. But, even in this pump, as in the thread-type pump, the gas flow passage extends from the inlet side toward the outlet side along the winding direction of the spiral so that as in the thread-type pump, the pumping speed is extremely reduced in the range in which the compression ratio of the turbo-molecular pump is reduced.